Actually what I would do is keep my altimeter on the QNH (for terrain clearance) and read my FLs (for traffic separation) from the transponder. It's a useful trick my aerobatics instructor showed me when we were doing aerobatics below and above the TA/TL.
Have to have a mode-C transponder at least for that to work though. And of course you need to be aware of what the TA/TL actually is, so you know whether to read back the altimeter (altitude) or transponder (FL) back to ATC.
For IFR flying obviously you need to follow IFR procedures though.